The ever growing popularity of environmentally friendly or “green” products and services has led to a renewed interest in the electric car. While the practical, fully electric powered car might still be a few years in the future, its cousin, the hybrid car is already being used by the general public. Both these types of vehicles have their batteries usually recharged by plugging into regular 120V or 220V wall sockets. Developments that affect battery charging systems would therefore have a great impact on these types of vehicles.
Electric Vehicle (EV) power conditioning systems usually utilize a high energy battery pack to store energy for the electric traction system. A typical block diagram of the power conditioning system in an EV is shown in FIG. 1. The high energy battery pack is charged from a utility AC outlet. This energy conversion during the battery charging is performed by an AC/DC converter. Such AC/DC converters include a front-end boost converter, which performs input power factor correction (PFC) and AC/DC conversion, and a full-bridge DC/DC converter, for battery charging and galvanic isolation. PFC is useful for improving the quality of the input current, which is drawn from the power utility so as to comply with the regulatory standards like IEC1000-3-2.
Switching losses of the power switches in boost PFC AC/DC converters significantly deteriorate the efficiency of the converter. Present products usually use active auxiliary circuits in order to provide soft-switching, which increases the complexity of the system while decreasing its reliability.
One technique which can reduce switching losses is zero voltage switching. Most converters use MOSFETs (metal oxide semiconductor field effect transistors) in low to medium power applications (i.e. application involving a few kilowatts). In order to have robust and reliable operation, MOSFETs are preferably switched under zero voltage. Operating at Zero Voltage Switching (ZVS) decreases the converter switching losses and provides a noise free environment for the system control circuit. Loss of ZVS means extremely high switching losses at high switching frequencies and very high EMI. Loss of ZVS can also cause a very noisy control circuit, which leads to shoot-through and loss of the semiconductor switches.
There is therefore a need for solutions which lessen switching losses without the drawbacks of the prior art.